Several methods have been developed for creating thin films on substrates used in manufacturing semiconductor devices. Among the more established techniques is Chemical Vapor Deposition (CVD). Atomic Layer Deposition (ALD), a variant of CVD, is a relatively newer technology now emerging as a potentially superior method of achieving uniform, conformal film deposition.
ALD has demonstrated an outstanding ability to maintain ultra-uniform thin deposition layers over complex topology. This is at least partially true because ALD is not as flux dependent as is CVD. This flux-independent nature of ALD allows processing at lower temperatures than with conventional CVD methods.
The technique of ALD is based on the principle of the formation of a saturated monolayer of reactive precursor molecules by chemisorption. A typical ALD process for forming an AB film, for example, on a substrate consists of injecting a precursor or reactant A (RA) for a period of time in which a saturated monolayer of A is formed on the substrate. Then, the precursor or reactant A (RA) is purged from the chamber using an inert gas, GI. This is followed by injecting precursor or reactant B (RB) into the chamber, also for a period of time, to combine B with A thus forming the layer AB on the substrate. Then, the precursor or reactant B (RB) is purged from the chamber. This process of introducing precursor or reactant A (RA), purging the reactor, introducing precursor or reactant B (RB), and purging the reactor can be repeated a number of times to achieve an AB film of a desired thickness.
However, conventional ALD processes suffer from several drawbacks. Since the film is created one layer at a time, film growth is much slower than CVD, often by an order of magnitude. This has a significant negative impact on process throughput.
In-situ measurements indicating when saturation of a precursor monolayer is completed on the substrate(s) are not available; this hinders the ability to control and optimize processing conditions to achieve optimal performance and throughput.
For typical batch reactors, single orifice injectors are used for introducing precursors into the reactor; this approach takes a longer time to fill the reactor with the precursors and hence may take a longer time to create the saturated monolayer on the multiple wafers located in the reactor.